No description available.

No description available.

Early studies of mid-ocean ridges suggest a fundamental difference between crustal accretionary processes at slow- and fast-spreading ridges. Accretion, and the supply of melt to the crust itself, is thought to be highly episodic at slow-spreading ridges but steady-state at fast-spreading ridges. However, recent studies are beginning to question this model, with evidence for the temporal variation in crustal accretionary processes at all spreading rates emerging. This study provides evidence from bathymetry, TOBI sidescan, gravity and magnetic data, collected during different cruises to the Reykjanes Ridge, for the temporal nature of crustal accretion and its relationship to segmentation. Interpretation of TOBI images indicates that individual adjacent axial volcanic ridges (AVRs) vary in relative age, suggesting that they are at various stages of an evolutionary lifecycle, with episodic cycles of magmatic and tectonic activity. However, prior to investigating the possible effects of tectonomagmatic cycles on the crustal structure of AVRs, the effect of the Iceland hotspot on the ridge is examined. The along-axis free-air gravity anomaly is forward modelled in 2-D, revealing an along-axis increase in crustal thickness towards Iceland from 7.5 km to 10.5 km and a decrease in mantle densities from 3.30 to 3.23 g cm"^ between 57 30'N and 62 N. Calculation of the residual mantle Bouguer Anomaly (RMBA) and inversion of magnetic anomaly data, reveal intermediate-wavelength fluctuations in RMBA amplitude and magnetization intensity respectively that are attributed to hotspot pulses, with 59 N marking the southern most extent of the most recent pulse. Removal of the hotspot effect on the gravity data reveals short-wavelength RMBA lows, associated with individual AVRs, superimposed on a broad ridge-trending low. Along-AVR-axis gravity modelling shows that a number of these RMBA lows can be explained by a 200-800 m thickening of the crust and/or by the presence of 5-20% partial melt in the mid-crust. A correlation between relative AVR age and crustal structure is established, with longer, more mature AVRs having a thicker crust and shorter, younger AVRs having more partial melt in the mid-crust. Short-wavelength magnetization intensity highs, associated with younger AVRs, corroborate the TOBI age interpretations. Local spreading rate calculations reveal that total spreading rates for younger AVRs are up to 20% faster than for older AVRs over the last 1.42 Ma. On the basis of these results a model for the cyclicity of crustal accretion is presented, whereby far-field tectonic stresses result in spreading-orthogonal brittle deformation of the crust in the neovolcanic zone, and 3-D mantle upwelling, with a wavelength of -70 km, follows the ridge trend and results in second order segments that comprise ~5 AVRs. It is proposed that along-axis migration of melt within such a segment results in the observed variations in AVR age, length, RMBA amplitude, magnetization intensity and local spreading rate. The proposed model has implications for the temporal variability of crustal accretion at all spreading rates.

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Carnegie Ridge is a prominent bathymetric feature of the Gal??pagos Volcanic Province originated from the interaction of the Gal??pagos Hot Spot and the Cocos - Nazca Spreading Center. Our present knowledge regarding its crustal structure is limited to ridge transects along which wide-angle refraction seismic experiments have been conducted. In this study, the long-wavelength crustal structure of Carnegie Ridge between ~81?? W and 89?? W was determined by employing 2-D forward gravity modeling as the primary analytical technique. Model structures were built by assuming Airy isostasy and crustal layers of constant density. The geometry and density structure of the thickened oceanic crust beneath the ridge was constrained based on available seismic velocity models. Except for regions near the Ecuador Trench, the gravity modeling solution along the different transects examined in this study accounted adequately for the observed gravity anomaly field over the ridge. Crustal overthickening mainly accommodated in oceanic layer 3 and the asymmetry of the crustal root geometry characterize the estimated long-wavelength crustal structure. The asymmetry on eastern Carnegie Ridge is thought to be related to a ridge-related rifting whereas the origin and nature of that estimated on western Carnegie Ridge remain uncertain. Crustal volume fluxes were calculated at Carnegie Ridge and the Gal??pagos Archipelago, and at Cocos Ridge in order to explain the along-axis variations of the estimated crustal thickness. Along-axis crustal thickness variations on eastern Carnegie Ridge, and the formation of its bathymetric saddle were found to be related to the decline in the total volume output of the Gal??pagos Hot Spot. According to my results, this decay started soon after the spreading center shifted to the south of the hotspot (i.e., ~15 Ma) and continued for ~4.5 m.y. Since ~10 Ma the volume output of the GHS started to increase again, giving rise to the formation of western Carnegie Ridge and the Gal??pagos Archipelago. This increase continued until ~2 Ma, when the hotspot intensity started a new decrease that continues until the present time.

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Deep troughs and ridges of the Arctic Basin are some of the least known features of the Earth's crust. Some of the ridges, eg. Chukchi and Nordwind, are connected directly to the continental shelves and are certainly submarine promontories of the latter. The character of the Lomonosov Ridge as a narrow slice of continental crust that separated from the Eurasian margin in the early Cenozoic (by opening of the Eurasian Basin), is not in doubt. Recent drilling (ACEX) and piston coring have confirmed this interpretation. However there are many other ridges and some of the troughs that are of uncertain origin. Seismic research in combination with potential field data over the East-Siberian margin, Podvodnikov and Makarov basins and the Mendeleev Ridge, presented here, provides a framework for understanding this enigmatic part of the Earth. The constrained models of the crust illustrate their structure. The crust beneath the East Siberian margin is up to 40 km thick; it thins to about 20 km towards to the Podvodnikov Basin. The models over the Arlis Gap, in the middle of the Podvodnikov Basin, and the Mendeleev Ridge have shown that the crust beneath both these features is anomalously thick (up to 28–32 km) and has a velocity structure that suggests the presence of highly attenuated continental crust. The crustal thickness over the Makarov Basin varies from 8 km to 15 km. Reflection profiles provide evidence of the character and thickness of the sedimentary cover (mostly Cenozoic and late Mesozoic), both on the ridges and beneath the troughs. Presented here is evidence that some of the ridges (eg. Marvin Spur) appear to be fragments of continental crust rifted off the Lomonosov Ridge (with a similar, unconformable Cenozoic cover); however, they gently plunge into and beneath troughs (eg. Makarov Basin). Reflection seismic data collected by the HOTRAX expedition in 2005 over the central part of the Lomonosov Ridge illustrate the sedimentary structure on the top of the Ridge and in an internal basin. The main sedimentary units can be interpreted by correlation with the ACEX results. The major fault separating the surrounding ridges from the internal basin appears to have a roll over anticline in the hanging wall, suggesting that the basin was created by a growth fault. The seismic lines provide evidence of gently folded basement beneath the Lomonosov Ridge with intra basement reflections are usually parallel to the upper surfaces; in combination with velocities (c. 4–5 km/s), these suggest the presence of old well-consolidated sediments.

Passive tectonic margins, like the eastern North American margin (ENAM), represent the meeting of oceanic and continental material where no active deformation is occurring. The recent ENAM Community Seismic Experiment provides an opportunity to examine the crustal structure across the ENAM owing to the simultaneous deployment of offshore and onshore seismic instrumentation. Using Rayleigh wave phase and group velocities derived from ambient noise data, we invert for shear velocity across the ENAM. We observe a region of transitional crustal thicknesses that connects the oceanic and continental crusts. Associated with the transitional crust is a localized positive gravitational anomaly. Farther east, the East Coast magnetic anomaly (ECMA) is located at the intersection of the transitional and oceanic crusts. We propose that underplating of dense magmatic material along the bottom of the transitional crust is responsible for the gravitational anomaly and that the ECMA demarks the location of initial oceanic crustal formation.

In this thesis, a finite-frequency theory is developed to calculate Born sensitivity kernels for Rayleigh-wave phase and amplitude measurements that are valid in regions near seismic stations. Calculations of sensitivity kernels for inter-station measurements show that exact travelling-wave representation of Green tensor is necessary when station spacing is close to or smaller than the seismic wavelength. This finite-frequency theory will allow us to take advantage of dense seismic arrays to obtain high-resolution surface-wave tomography using inter-station measurements. The non-linear dependence of surface wave phase upon large perturbations in crustal thickness as well as finite-frequency effects in global surface-wave tomography are investigated using wave propagation simulations. Calculations show that non-linearity as well as finite-frequency effects can be accounted for by using 2D phase-velocity kernels for boundary perturbations. A 3D-reference tomographic approach is developed for iterative inversions of global crustal structure where Frechet kernels are calculated in 3D reference models. A global dataset of minor-arc and major-arc Rayleigh wave dispersion measurements at periods between 25 seconds and 100 seconds are built and global phase velocity maps based on the dataset are obtained using diffractional tomography. The phase velocity model confirms many general features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. There are significant differences between the phase velocity model and calculations based on a current global model CRUST2.0+S20RTS in oceanic regions, Archean and Proterozoic cratons as well as orogenic belts. In addition, the high resolution phase velocity maps reveal a major change in the distribution of small scale anomalies in the Pacific at different wave periods. / Ph. D.

Controlled source seismic methods were employed in this study to investigate the reflectivity and velocity structure of two Hercynian orogens – the Uralides and Variscides. Conventional common depth point (CDP) sections from five reflection seismic campaigns and a velocity model obtained from tomographic inversion of wide-angle observations were the main datasets studied from the Middle Urals. These were complemented with the near-vertical seismic sections and velocity models from the Southern Urals. In the Variscides, conventional CDP processing, along with non-standard processing and synthetic data modeling, were used to obtain and interpret reflection seismic images of the Southwestern Iberian crust. Although, the Uralian and Variscan belts were formed in Late Paleozoic time in apparently similar plate collisional settings, a comparison of the seismic results show that the crust of these two orogens looks quite different at depth. In the Urals, collision of Baltica with Asian terranes (Siberia and Kazakhstan) resulted in a highly diversely reflective crust of 40-45 km thickness. The axial zone of the orogen is characterized by a high velocity crustal root of diffuse reflectivity and an imbricated Moho, with a crustal thickness reaching 55-60 km. The Moho discontinuity is marked by a sharp decrease in reflectivity and is well imaged in most locations except in the crustal root zone. The Southwestern Iberian Variscan crust is 30-35 km thick and is characterized by a highly reflective two-layered structure that resulted from collision of Luarussia and Gondwana, including terranes in-between them. This type of crustal structure is very similar to those imaged in other regions of the Variscan belt in the Europe. The Moho discontinuity is flat and appears to be the deepest reflection. This thesis compares the deep structure of the two orogens and interprets mountain building processes related to late Paleozoic plate movements.